In one widely used apparatus for magnetic recording and/or reproducing of video signals, a magnetic tape is wrapped helically around a guide drum and driven, by a capstan and a pinch roller, from a supply reel to a take-up reel. The drum has a slot at right angles to its axis. Two recording or reproducing heads respectively rotate in the slot around this axis, scanning the magnetic tape in oblique or diagonal parallel lines. Usually, one video field or one video frame is recorded on one track with the vertical synchronizing signal in the neighborhood of the transition from one track to the next. Adjacent one edge of the tape, a control track is recorded or scanned by a stationary control head. During playback, the control pulses scanned by the head are compared to tachymetric or position pulses generated by the video head wheel motor, to make sure that the recording and reproducing heads exactly follow the tracks.
This can be accomplished when and only when, during the recording mode and during the reproducing mode, the tape length between the gap of the control head and the beginning of a track is substantially the same. This condition cannot, however, be fulfilled, partly because of mechanical tolerances between different machines, and partly because of different tape elongations under the influence of different tape tension or temperature. Therefore, ordinary tape recorders are equipped with a knob to manually adjust and readjust the phase of the control signals and to tune to the optimum signal-to-noise ratio. This procedure is not only cumbersome, but also difficult for laymen.
To overcome these difficulties, several automatic track searching devices have been proposed. In German Patent Application No. 2,116,206, which has been laid open to public inspection, for example, during playback, the tape transportation speed is slowly and periodically modulated which results in an amplitude modulation of the scanned video signal. The amplitude modulation is then detected and used to readjust the position of the rotating video heads. This method, however, is very expensive and cannot be employed if the video signal itself holds or contains some amplitude modulated information, as is usually the case with color video recordings. Furthermore, the velocity modulation deteriorates the quality of the sound track and requires certain limitations.
In another method (German Patent Application 2,048,633 which has been laid open to public inspection) during recording, special pulses are inserted into the trailing porch of the line synchronizing signals which can be extracted during playback and used to control the servo mechanism of the motors. This method, too, is expensive and fails obviously in the overwhelming plurality of tapes not specially prepared during recording.